What I do vs. Corrin (it usually works out)
- Don't side-B excessively. This is a bad idea vs any character because of how easily telegraphed and punished the move can be, but it's really detrimental against Corrin, who can side-B you for 19% and get over 20% from one string off of a nair or fair. I only side-B to react to what he does or to fake him out by jumping.
- Don't get faired. Part of what makes Corrin dangerous is how much he gets off of fair. He can rack up big damage off of fair and, at later percents, combo fair > uair for a kill. If you're going to attack him with your own fair or bair, try to beat him to the punch by hitting him with your aerial before he hits you. Note that your fair has similar range to his, but his is faster.
- Gimp/edgeguard the hell out of him. Have a mech in your hand when he has to recover low and throw it down at him as he up-B's. At high percents, he won't come back unless he uses the propulsion from his bair, but you can kill him outright by comboing the downward mech throw into a fair or a side-B spinout. The spinout kills earlier than the fair does.
- Take him to a stage with bigger blast zones. Your increased lifespan against his side-B, up-throw, and other attacks will help a lot.
- Take note of the fact that Corrin can counter your mech as it's walking. Some Corrins will use this often. (off-topic, but also take note of the fact that Bayonetta can Witch Time you with your own mech)
TL;DR don't take excessive damage, but don't let him walk all over you either. Assert yourself in the neutral and maintain stage control with your mechs.